From patchwork Wed Mar 17 14:59:51 2010
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: [QEMU] Transparent Hugepage Support #3
From: Andrea Arcangeli
X-Patchwork-Id: 47947
Message-Id: <20100317145950.GA5752@random.random>
To: qemu-devel@nongnu.org
Cc: Avi Kivity , Paul Brook
Date: Wed, 17 Mar 2010 15:59:51 +0100
From: Andrea Arcangeli
This will allow proper alignment so NPT/EPT can take advantage of
linux host backing the guest memory with hugepages. It also ensures
that when KVM isn't used the first 2M of guest physical memory are
backed by a large TLB. To complete it, it will also notify the kernel
that this memory is important to be backed by hugepages with madvise
(needed for both KVM and QEMU) so that hugepages can also be used in
embedded systems without any memory waste and in the future it will
allow khugepaged to prioritize the collapse of hugepages into the
madvise regions.
Ideally the max hugepage size provided by the transparent hugepage
support in the kernel should be exported by some sysfs file, but
there is no reason to expect x86_64 host to have hugepages larger than
2M or to expect those to be supported by the kernel transparent
hugepage support in the short and medium term, so we can defer the
invention of a fixed kernel API until this happens, by that time we'll
surely have a better clue of what's the best way to provide that
information to userland and it'll be a few liner change to adapt qemu
to use it so there's no hurry to do it right now. Plus the below will
keep to remain optimal and there is no risk of memory waste as virtual
memory is practically zero cost on 64bit archs.
NOTE: if the callers of qemu_ram_alloc changes significantly we may
later be required to pass a second parameter to qemu_ram_alloc that
will tell it what is the first guest physical address that corresponds
to the "sized" memory block being allocated. I'd defer this change for
later too as it may never be needed.
I verified this is more than enough to get the max benefit from the
kernel side feature.
cat /sys/kernel/debug/kvm/largepages
301
Signed-off-by: Andrea Arcangeli
diff --git a/exec.c b/exec.c
index 14767b7..ab33f6b 100644
--- a/exec.c
+++ b/exec.c
@@ -2745,6 +2745,18 @@ static void *file_ram_alloc(ram_addr_t memory, const char *path)
}
#endif
+#if defined(__linux__) && defined(__x86_64__)
+/*
+ * Align on the max transparent hugepage size so that
+ * "(gfn ^ pfn) & (HPAGE_SIZE-1) == 0" to allow KVM to
+ * take advantage of hugepages with NPT/EPT or to
+ * ensure the first 2M of the guest physical ram will
+ * be mapped by the same hugetlb for QEMU (it is worth
+ * it even without NPT/EPT).
+ */
+#define PREFERRED_RAM_ALIGN (2*1024*1024)
+#endif
+
ram_addr_t qemu_ram_alloc(ram_addr_t size)
{
RAMBlock *new_block;
@@ -2768,11 +2780,19 @@ ram_addr_t qemu_ram_alloc(ram_addr_t size)
PROT_EXEC|PROT_READ|PROT_WRITE,
MAP_SHARED | MAP_ANONYMOUS, -1, 0);
#else
- new_block->host = qemu_vmalloc(size);
+#ifdef PREFERRED_RAM_ALIGN
+ if (size >= PREFERRED_RAM_ALIGN)
+ new_block->host = qemu_memalign(PREFERRED_RAM_ALIGN, size);
+ else
+#endif
+ new_block->host = qemu_vmalloc(size);
#endif
#ifdef MADV_MERGEABLE
madvise(new_block->host, size, MADV_MERGEABLE);
#endif
+#ifdef MADV_HUGEPAGE
+ madvise(new_block->host, size, MADV_HUGEPAGE);
+#endif
}
new_block->offset = last_ram_offset;
new_block->length = size;